We need more information than that.
Random, or specific recipients? Can you receive texts from the same people? Ordinary texts or some other sort of message (e.g. MMS)? One on one or group messaging? Can affected recipients receive messages from others? Is there any other common factor between people who can't receive your texts?
Bear in mind that non-delivery of texts could be a problem with your phone or app, their phone or app, your carrier, their carrier or communication between the two. That's a lot of options, so to help at all we need enough information to allow the options to be narrowed a bit.